1. Field of the Invention
The present invention relates to a filter fault detection apparatus for a filter for collecting particulate matter contained in exhaust gas discharged from an internal combustion engine.
Japanese Patent No. 5115873 describes a filter fault detection apparatus for a filter for collecting particulate matter) contained in exhaust gas discharged from an internal combustion engine. This filter fault detection apparatus uses a sensor of the electric resistance type that outputs an electrical signal having a magnitude depending on the amount of particulate matter contained in exhaust gas. The sensor of the electric resistance type includes a sensor element having a pair of electrodes between which a voltage is applied. Since the particulate matter to be filtered contains carbon particles having electrical conductivity, when the amount of particulate matter collected between the electrodes of the sensor element exceeds a certain value, the electrodes become electrically conductive therebetween, and the sensor outputs an electrical signal having a magnitude depending on the amount of collected particulate matter.
The sensor is disposed downstream of a filter to be monitored. The filter fault detection apparatus including this sensor detects whether the filter is faulty or not based on the conduction start timing (the timing at which the electrical signal outputted from the sensor rises). More specifically, this filter fault detection apparatus predicts, as a reference timing, the timing at which the sensor output (the electrical signal outputted from the sensor) will rise on the assumption that the filter is a reference filter for detection (referred to as the “reference fault filter” hereinafter). If the actual timing at which the sensor output has risen actually is earlier than the reference timing, it is determined that the filter is faulty.
However, the above fault detection apparatus has a problem in that it may make an erroneous determination if the sensor output rises under unexpected circumstances. One example of the unexpected circumstances is that the collected particulate matter comes off the filter or an exhaust gas passage as a lump, and adheres to the sensor element. If an erroneous determination is made by the fault detection apparatus, a MIL (malfunction indicator light) lights causing unnecessary replacement of the filter.